Pigging is used as a mechanism to clean internal surfaces of pipelines including sewer, process, water, wastewater and other types of pipes. Pigging is used to reduce the friction losses of pumping energy, improve flow rates of restricted piping, and as a general maintenance process. Typical materials of construction for water pipe and municipal, commercial, and industrial sewer pipe include ductile iron, polyvinyl chloride (“PVC”), and lined ductile iron or cast iron pipe. As sewer force mains are used over time, the flow of sewage is restricted by a buildup of restricting materials on the inside of the pipe. This material includes solid waste, grease, pipe corrosion and other materials. In addition to these restrictions, force sewer mains, which were installed years and sometimes generations ago, are used to carry flow in excess of their original design capacity. As the pipe is restricted over time, the pumping horsepower requirement increases, flow is reduced, and electrical energy consumption of the pump is increased.
Pigging is the process of inserting a device (known commercially as a pig) into the pipe through an insertion point installed at a particular location in the pipe. The insertion point is generally referred to as a launch station, and includes an array using a tee, valving, and a pressure injection point to allow the pig to be inserted into the pipe and then propelled through the pipe via motive force which may be water, steam or compressed air. Once inserted, the pig is propelled through the length of pipe, which is typically underground, to a designated removal point. While traversing the pipe the pig scours the inner wall of the pipe, without damaging the pipe, to remove any flow restricting material.
Pig construction is generally of polyurethane foam, with or without an abrasion resistant cover. Alternate pig materials of the prior art include Styrofoam, polypropylene, and ice. Various abrasive coatings may also be used and brushes may be adhered or fastened to the bearing surfaces (where the pig is in contact with the pipe wall). These abrasive coatings and brushes aid the pig in clearing debris from the interior of the pipe.
Currently, pigging utilizes a process of progressive pigging, wherein several pigs of varying diameters, all being smaller in overall outside diameter than the nominal inside diameter of the pipe, are used to insure that no obstruction which would cause the pig's progress though the pipe to be impeded to the point of the pig being stalled or stuck in the pipe. Once a smaller diameter pig has successfully traversed the length of pipe being cleaned, a slightly larger pig is inserted, and the process repeated until near nominal internal pipe diameter is reached, or obstruction of the pipe becomes a risk. During the pigging process the pressure of the propulsion medium is monitored continuously. So long as the pig is traveling through the pipe, the pressure remains at a constant level, plus or minus a predetermined tolerance. As fluid or gas is injected into the pipe, the pig travels through the pipe increasing the effective volume filled by the propulsion media, resulting in relatively constant pressure. If the pig movement stops, the volume behind the pig within the pipe no longer increases, and as propulsion material is added the pressure within the pipe will rise rapidly.
A stalled or stuck pig represents a significant technical and logistical problem. The overwhelming majority of wastewater and sewage piping is installed underground. Based on the age and location of the pipe, as well as potential interference with other utility piping, the effect on other electrical and telecommunication utilities by a stuck or stalled pig is largely unknown. Even in a scenario where the exact pipe location is documented, the exact pig location within the pipe may still be unknown.
As a result, the location and retrieval of a stalled pig can be incredibly costly. Often, the retrieval requires a utility shutdown, a diversion of sewage via multiple vacuum trucks at sewage interceptors, and coordination with municipal and utility entities to divert traffic flow and coordinate utility shutdowns while excavation and removal of the stuck pig occurs. In such a case, the only method to remove a stalled pig is to begin excavating the pipe. Due to the fact that the exact location of the stuck pig is unknown, multiple excavations may be required until the exact location of the stalled pig is determined and successful removal of the pig is achieved.
As an alternative to progressive pigging, and in an attempt to mitigate the risk and cost of a stalled or suck pig, a process of pigging has been modified to use pigs made of ice or gelatin. Pigs made of these materials will degrade and break down as the pig travels the length of the pipe. This modification substantially eliminates the risk of a stalled pipe since the pig will melt or erode as it passes through the pipe. Pigs made of ice will continue to melt even after the pig becomes stalled or stuck in the pipe. The erosive nature of pigs made of these materials often eliminates the problem with no need for excavation of the pipe or other remedial action. Unfortunately, the overall dimension of the ice or gelatin pig begins to decrease as soon as the pig is inserted into the pipe. As the pig traverses the length of the pipe, the reduction in overall dimension continues, resulting in a reduction in effectiveness of the ice or gelatin pig as the pig travels the length of the pipe. Ultimately, use of an ice or gelatin pig for long distance pigging is of very little if any benefit.
There exists a need for a novel pig design that substantially reduces the risk of the pig being stuck or stalled in a pipe and which can be retrieved without substantial additional risk or cost. It would also be advantageous if the novel pig design improved and maintained the effectiveness of the pig as it travels the length of the pipe, thereby improving current methods for pigging of long distance pipelines.